Endothermic vs. Exothermic Reactions – Difference 

1.0Introduction 

Understanding the difference between endothermic and exothermic reactions is fundamental in Chemistry, especially for students preparing for competitive exams like JEE. Chemical reactions are not just about the transformation of reactants into products; they also involve significant energy changes that determine how and why reactions occur. These energy changes can either be in the form of absorption or release of heat, which is why reactions are classified as endothermic or exothermic.

In an endothermic reaction, energy is absorbed from the surroundings, leading to a decrease in the surrounding temperature. In contrast, an exothermic reaction releases energy, usually as heat, causing an increase in the surrounding temperature. These concepts play a crucial role in various real-life applications, from biological processes like photosynthesis and respiration to industrial processes like combustion and thermal decomposition.

2.0What are Endothermic Reactions?

An endothermic reaction is a chemical process that absorbs energy from its surroundings, usually in the form of heat. In these reactions, the energy required to break the bonds of the reactants is greater than the energy released when new bonds are formed in the products.

Characteristics of Endothermic Reactions

• Absorption of Heat: Reactants absorb energy from the surroundings.
• Decrease in Surrounding Temperature: The immediate environment feels colder.
• Positive Enthalpy Change (ΔH > 0): Enthalpy, a measure of total energy, increases.
• Non-spontaneity: Most endothermic reactions do not proceed spontaneously without an external energy source.
• Bond Breaking Dominates: Energy is consumed primarily in breaking chemical bonds.

Examples of Endothermic Reactions

1. Photosynthesis: Plants absorb sunlight to synthesize glucose, a classic endothermic reaction.

• $6C{O}_2+6H_{2}O+\text{energy (sunlight)}\to C_6{H}_{12}{O}_6+6O_2$

2. Thermal Decomposition: Calcium carbonate decomposes into calcium oxide and carbon dioxide upon heating.

• $CaC{O}_3(s)+\text{heat}\to CaO(s)+C{O}_2(g)$

3. Dissolution of Certain Salts: Dissolving ammonium nitrate ((NH4NO3)) in water results in cooling because energy is absorbed.

Energy Profile Diagram for Endothermic Reactions

The energy profile for an endothermic reaction shows that the products have higher energy than the reactants. The curve rises, indicating energy absorption, and the difference between products and reactants is the positive enthalpy change ((ΔH).

3.0What are Exothermic Reactions?

An exothermic reaction is a chemical process that releases energy (usually as heat) into the surroundings. The energy released from forming product bonds is greater than the energy required to break the reactant bonds.

Characteristics of Exothermic Reactions

• Release of Heat: Surroundings feel warmer as heat is given out.
• Increase in Surrounding Temperature: The temperature rises during the reaction.
• Negative Enthalpy Change (ΔH < 0): The system’s enthalpy decreases.
• Spontaneity: Many exothermic reactions occur spontaneously.
• Bond Formation Dominates: Energy is released mainly during the formation of new bonds.

Examples of Exothermic Reactions

1. Combustion of Fuels: Burning of methane releases heat and light.

• $C{H}_4+2O_2\to C{O}_2+2H_{2}O+\text{energy}$

2. Neutralization Reactions: Acid-base reactions typically release energy.

• $HCl+NaOH\to NaCl+H_{2}O+\text{energy}$

3. Respiration: Cellular respiration releases energy stored in glucose.

• $C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H{2}_O+\text{energy}$

Energy Profile Diagram for Exothermic Reactions

The energy profile for exothermic reactions shows that products have lower energy than reactants. The curve descends, indicating the release of energy, and the difference corresponds to a negative enthalpy change (ΔH).

4.0Key Differences Between Endothermic and Exothermic Reactions

5.0Significance of Exothermic and Endothermic Reactions in Chemistry and Daily Life

• Industrial Relevance: Exothermic reactions are crucial in manufacturing (e.g., energy production, metallurgy) while endothermic reactions are used in cooling applications and chemical synthesis.
• Biological Importance: Photosynthesis (endothermic) and respiration (exothermic) are vital for life cycles.
• Everyday Examples: Instant cold packs (endothermic), hand warmers (exothermic), cooking and baking, fuel burning, etc.